This invention relates to gain-controlled amplifiers, and is more particularly directed to a solid-state image pickup device incorporating a gain-controlled current amplifier for converting a relatively feeble video signal to a strong video output signal with a high signal-to-noise (S/N) ratio.
A solid-state image pickup element, such as an MOS imager, generally provides a quite weak signal which must be amplified before being applied to a successive stage. This weak video signal is particularly susceptible to noise due to stray currents, shot noise, and the like. In an MOS imager, the video signal is provided as a differentiated current. This signal must be integrated to obtain an average value, but integration lowers the overall level of the video signal. Thus, the integrated signal has a rather low S/N ratio.
An external amplifier is conventionally provided to amplify the output signal and thereby convert it to a level suitable for further processing. Unfortunately, this external amplifier amplifies the noise along with the video signal, and, because it is external, also introduces noise. Consequently, the resulting video output signal has a further deteriorated S/N ratio.
Furthermore, the average level of the output signal from the image pickup element varies greatly with changes in brightness or color temperature of a scene to be televised. If the gain of the circuit connected to the pickup element is too high, the peak level becomes clipped and the signal waveform becomes distorted when a bright scene is being shot. As a result, the picture becomes washed out, and lacks detail in its bright portions. On the other hand, if the gain of the circuit is too low, the average signal level is reduced and the S/N ratio can become unacceptably low when a dark scene is shot. Obviously, both of the foregoing situations are undesirable. Consequently, it would be advantageous to include a gain control element in the video signal processing circuitry so that the gain of the output circuitry associated with the pickup element changes in accordance with the brightness of the picture being picked up.
Unfortunately, conventional gain-controlled amplifiers generally are suitable only for amplifying a continuous analog signal, and no such circuit has been previously proposed which can control the amplitude of a sampled, time-shared current signal such as the output of an MOS imager or other similar image pickup device.